1. Field of the Invention
The present invention relates to the field of deposition processes and more specifically by gaseous deposition processes for semiconductor device fabrication.
2. Prior Art
Modern integrated circuits are fabricated utilizing a plurality of oxidizing, masking, etching, diffusion and deposition processes. Because of the selective operations, particularly etching operations after the masking of the required detailed pattern, the topography of the initially flat wafer-being processed becomes sharply defined in very fine detail by the minute, though clearly defined layers and/or levels developed in or on the semiconductor wafer. In certain instances during processing it may be desired to create a silicon oxide layer for insulation and/or passivation purposes. Where the oxide layer is to be created on the semiconductor surface the semiconductor surface can be oxidized by heating in an oxygen rich environment. However, this requires a substantial temperature and of course a semiconductor surface to be oxidized, whereas in certain instances the exposed semiconductor surface is not present, and the development of the integrated circuit has proceeded to a stage where high temperature exposure is not possible or desirable. In particular, once metalization layers are in place, particularly aluminum metalization layers such as for connecting pads, interconnect lines, metal gates, etc. The maximum tolerable temperature for the semiconductor wafer is quite limited. Further, of course, a silicon oxide layer cannot be grown over the metalization layers for passivation purposes but must be deposited by some suitable process within the allowable temperature exposure capabilities of the wafer at that stage of fabrication.
One process for the deposition of silicon oxide which is known in the prior art is the decomposition of silane (SiH.sub.4) in an oxygen rich environment to produce silicon dioxide and water in accordance with the following reaction: EQU SiH.sub.4 +20.sub.2 .fwdarw.SiO.sub.2 .dwnarw.+2H.sub.2 O (1)
This process as well as equipment for practicing the process is disclosed in U.S. Pat. No. 3,854,443.
For the efficient use of this process on a production basis, it is highly desirable if the process may be practiced in a batch processing scheme, utilizing production ovens and furnaces commonly found in semiconductor manufacturing facilities. Such equipment allows the processing of many wafers at the same time, so that oxidation, diffusion, etc. may be done on a batch basis rather than an individual basis. Such furnaces generally have a cylindrical configuration, with the inner furnace wall being defined by a quartz tube closed at one end and having a door on the other end, within which boats supporting semiconductor wafers can be inserted for the batch processing. However, when attempts are made to utilize such equipment for the deposition of silicon oxide by the decomposition of silane, substantial non-uniformity in the deposition rates are found, both across the area of any individual wafer, and more particularly along the length of the over because of the drop in the concentration of silane and variation in flow patterns along the length of the over from the injection region.
Deposition of silicon oxide by the decomposition of silane has been practiced in the past utilizing relatively low concentrations of silane in a nitrogen environment, at atmospheric pressure. Such a process provides the desired deposition, though has certain characteristics making it desirable to use a higher percentage concentration of silane in a much lower total pressure environment. However, problems regarding the uniformity of deposition rates across the face of an individual wafer and from wafer to wafer along the furnace tube are particularly severe when a low pressure process is attemped.
One prior art apparatus for the low pressure deposition of silicone oxide effectively uses a double-chambered furnace by the placement of a second enclosure within the first furnace enclosure, with the semiconductor wafers to be processed being located in the inner enclosure. The inner enclosure is coupled to a vacuum pump, and is appropriately perforated so that the atmosphere provided in the outer enclosure is effectively injected into the inner enclosure relatively uniformly along the length thereof. Thus, by providing the desired atmosphere in the outer enclosure and pumping on the inner enclosure, a distributed injection may be achieved. Such a system requires two relatively gas tight enclosures instead of only one. Also turbulence and injection shading as well as the directional characteristics of the flow established by the exhaust create certain limitations regarding this equipment.